The present invention relates to a test system comprising
(a) one or more identical or different immobilized nucleic acid(s) having at least one spliceable nucleic acid.
(b) at least one gel-free detection system for detecting a splicing reaction, where appropriate
(c) at least one composition comprising splicing components, and preferably
(d) suitable detection probes, and, where appropriate,
(e) further aids.
Most of the protein-encoding genes in eukaryotes are interrupted in their genomic form by one or more sequences not coding for the protein (introns). When transcribing the genomic DNA into messenger RNA (mRNA), these non-coding regions (introns) are incorporated into the primary transcript. In order to generate the correct mRNA, this pre-mRNA has to be processed.
The pre-mRNA is processed by removing the introns and fusion of the coding regions (exons). Only then is it possible to provide a nucleotide strand which can be read in an interrupted manner for translation in the cytoplasm. mRNA formation in eukaryotes therefore requires a xe2x80x9csplicing processxe2x80x9d in which the non-coding gene regions (introns) are removed from the primary gene transcript.
Splicing occurs in the nucleus, before the mRNA is transported out of the nucleus. It is generally carried out in a two-stage mechanism in which in each case a transesterification step is involved (Moore, J. M. et al., (1993) Splicing of precursors to mesenger RNAs by the Spliceosome. In The RNA world, Edited by Gesteland R. F., Gesteland, J. F., Cold Spring Harbor Laboratory Press, 303-358). The first step generates a free 5xe2x80x2 exon and a xe2x80x9clariat structurexe2x80x9d of the intron which is still linked to the 3xe2x80x2 exon. The lariat structure comprises a branched RNA which is produced by esterification of the 5xe2x80x2 end of the intron with a 2xe2x80x2-hydroxyl group of a ribose in an adenosine which is located approx. 20-40 nucleotides upstream of the 3xe2x80x2 end of the intron. The second catalytic step leads to ligation of the exons and liberation of the intron. Although no nucleotides are incorporated during these reactions, an energy source, for example ATP, is necessary for this catalysis (Guthrie, C. (1991) Science, 253, 157).
A plurality of factors is involved in the process of mRNA splicing. Two classes of splicing factors are distinguished at the moment. The first class comprises four evolutionarily highly conserved protein-RNA particles (small nuclear ribonucleoprotein particles=snRNPs); U1, U2, U4/U6 and U5, which comprise either one (U1, U2, U5) or two (U4/U6) snRNA components (Moore, J. M. et al., (1993) supra; Guthrie, (1991) supra; Green, M. R. (1991), Annu. Rev. Cell Biol., 7, 559). The second class comprises proteins which have not been characterized much up until now and which are not tightly bound to the snRNPs and are therefore called non-snRNP splicing factors (Lamm, G. M. and Lamond, A. J. (1993) Biochim. Biophys. Acta, 1173, 247; Beggs, J. D. (1995), Yeast splicing factors and genetic strategies for their analysis, In: Lamond, A. I. (ed) Pre-mRNA Processing Landes, R.G. Company, Texas, pp. 79-95. K xc3xa4mer A. (1995), The biochemistry of pre-mRNA splicing. In: Lamond, A. I. (ed), Pre-mRNA Processing. Landes, R.G. Company, Texas, pp. 35-64).
The composition of snRNPs has been studied most successfully in HeLa cells (Will, C. L. et al., (1995) Nuclear pre-mRNA splicing. In: Eckstein, F. and Lilley, D. M. J. (eds). Nucleic Acids and Molecular Biology. Springer Verlag, Berlin, pp. 342-372). At relatively low salt concentrations at which it is possible for nuclear extracts from HeLa cells to promote splicing of pre-mRNA in vitro, the snRNPs are present in a 12S U1 snRNP, a 17S U2 snRNP and a 25S [U4/U6.U5] tri-snRNP complex. At higher salt concentrations (approx. 350-450 mM) the tri-snRNP complex dissociates into a 20S U5 particle and a 12S U4/U6 particle. In the U4/U6 snRNP, the U4 and U6 RNAs are present base-paired via two intermolecular helices (Bringmann, P. et al. (1984) EMBO J., 3, 1357; Hashimoto, C. and Steitz, J. A. (1984) Nucleic Acids Res., 12, 3283; Rinke, J. et al., (1985) J. Mol. Biol., 185, 721; Brow. D. A. and Guthrie, C. (1988) Nature, 334, 213).
The snRNPs comprise two groups of proteins. All snRNPs comprise the group of general proteins (B/Bxe2x80x2, D1, D2, D3, E, F and G). In addition, each snRNP comprises specific proteins which are present only in said snRNP.
Thus, according to the prior state of research, U1 snRNP comprises three additional proteins (70K, A and C) and U2 snRNP eleven further proteins. According to prior knowledge, 20S U5 snRNP carries nine further proteins having molecular weights of 15, 40, 52, 100, 102, 110, 116, 200 and 220 kDa, while 12S U4/U6 snRNP comprises two additional proteins having molecular weights of approx. 60 and 90 kDa. 25S tri-snRNP [U4/U6.U5] comprises five additional proteins having molecular weights of approx. 15.5, 20, 27, 61 and 63 kDa (Behrens, S. E. and Lxc3xchrmann, R. (1991) Genes Dev., 5, 1439; Utans, U. et al., (1992) Genes Dev., 6, 631; Lauber, J. et al., (1996) EMBO J., 15, 4001; Will, C. L. et al. (1995), supra, Will, C. L. and Lxc3xchrmann, R. (1997) Curr. Opin. Cell Biol., 9, 320-328).
The composition of splicing components in Saccharomyces cerevisiae has not yet been studied in detail. Biochemical and genetic studies, however, indicate that the sequences of both the snRNAs and the snRNP proteins are evolutionarily highly conserved (Fabrizio, P. et al., (1994) Science, 264, 261; Lauber, J. et al., (1996), supra, Neubauer, G. et al., (1997) Proc. Natl. Acad. Sci. USA, 94, 385; Krxc3xa4mer, A. (1995), supra; Beggs, J. D. (1995); supra, Gottschalk, A. et al. (1998) RNA, 4, 374-393).
In order to form a functional splicing complex (spliceosome), the individual components (pre-mRNA, snRNPs and non-snRNP proteins) are combined in a stage-wise process. This is achieved not only by interactions of the pre-mRNA with the protein-containing components but also by numerous interactions between the protein-containing components themselves (Moore, J. M. (1993) supra; Madhani, H. D. and Guthrie, C. (1994) Annu. Rev. Genetics, 28, 1; Nilsen, T. W. (1994) Cell, 65, 115). The pre-mRNA sequence carries specific recognition sequences for the different splicing components. Firstly, U1 snRNP binds via said recognition sequences to the 5xe2x80x2 splicing region of the pre-mRNA intron. At the same time, an as yet unspecified number of various other factors (e.g. SF2/ASF, U2AF, SC35, SF1) is taken up by this complex and cooperates with the snRNAs in the continued formation of the pre-spliceosome. The U2 snRNP particle interacts with the xe2x80x9cbranch sitexe2x80x9d in the intron region (Krxc3xa4mer, A. and Utans, U. (1991) EMBO J., 10, 1503; Fu, X. D. and Maniatis, T. (1992) Proc. Natl. Acad. Sci USA, 89, 1725; Krxc3xa4mer, A. (1992) Mol. Cell Biol., 12, 4545; Zamore, P. D. et al. (1992) Nature, 355, 609; Eperon, J. C. et al. (1993) EMBO J., 12, 3607; Zuo, P. (1994) Proc. Natl. Acad. Sci. USA, 91, 3363; Hodges, P. E. and Beggs, J. D. (1994) Curr. Biol. 4, 264; Reed, R. (1996) Curr. Op. Gen. Dev., 6, 215). In a last step of spliceosome formation, [U4/U6.U5] tri-snRNP and a number of proteins not yet characterized in detail interact with the pre-spliceosome, in order to form the mature spliceosome (Moore, J. M. et al., (1993) supra).
For the splicing process, various interactions between pre-mRNA, snRNAs and sn-RNP are removed and new ones are formed. Thus it is known that before or during the first catalytic step of the splicing reaction two helices are separated from one another in the interacting structures of U4 and U6 and that new interactions form base pairs between U2 RNAs and U6 RNAs (Datta, B. and Weiner, A. M. (1991) Nature, 352, 821; Wu, J. A. and Manley, J. L. (1991) Nature, 352, 818; Madhani, H. D. and Guthrie, C. (1992) Cell, 71, 803; Sun, J. S. and Manley, J. L. (1995) Genes Dev., 9, 843). At the same time, binding of U1 to the 5xe2x80x2 splicing site is removed and pre-mRNA binds to the recognition sequence ACAGAG of U6 snRNA (Fabrizio, P. and Abelson, J. (1990), Science, 250, 404; Sawa, H. and Abelson, J. (1992) Proc. Natl. Acad. Sci. USA, 89, 11269; Kandels-Lewis, S, and Sxc3xa9raphin, B. (1993) Science, 262, 2035; Lesser, C. F. and Guthrie C. (1993) Science, 262, 1982; Sontheimer, E. J. and Steitz, J. A. (1993) Science, 262, 1989). U5 snRNP interacts via its conserved loop 1 with exon sequences which are located close to the 5xe2x80x2 and 3xe2x80x2 splicing sites. This process seems to be sequential, while the entire splicing process progresses from stage 1 to stage 2 (M. McKeown (1992) Annu. Rev. Cell Dev. Biol., 8: 133-155 Newman, A. and Norman, C. (1991) Cell, 65, 115; Wyatt, J. R. et al. (1992) Genes Dev., 6, 2542; Cortes, J. J. et al. (1993) EMBO J., 12, 5181; Sontheimer, E. J. and Steits (1993) supra). After the splicing reaction has finished, the mature mRNA is liberated and the spliceosome dissociates (Moore, J. M. et al (1993) supra).
Alternative splicing makes it possible to form from one and the same primary transcript various mature mRNAs which code for various proteins. In many cases, this alternative splicing is regulated. Thus it is possible to utilize this mechanism, for example, for the purpose of switching from a non-functional to a functional protein (e.g. transposase in Drosophila). It is furthermore known that alternative splicing is carried out tissue-specifically. Thus, for example, tyrosine kinase which is encoded by the src proto-oncogene is synthesized in nerve cells in a specific form by alternative splicing.
Incorrectly regulated or performed alternative splicing may lead to various conditions. In patients suffering from Graves"" disease it has been shown that incorrect splicing produces a crucial enzyme (thyroperoxidase) in an inactive form (Zanelli, E. (1990) Biochem. Biophys. Res. Comm., 170, 725). Studies of the disease spinal muscular atrophy indicate that a defective gene product of the SMN (survival of motor neurons) gene leads to a considerable disruption in the formation of snRNPs. Inhibition of the splicing apparatus of the motor neurons leads to paralysis of the nerve cells and to degeneration of muscle tissue (Fischer, U. et al. (1997), Cell, 90: 1023-9; Liu, Q. et al. (1997), Cell, 90: 1013-21; Lefebvre, S. et al. (1997) Nat. Genet. 16, 265). Particular alternative splicing variants of the membrane-bound molecule CD44, inter alia, seem to play a decisive part in cancer cell metastasis. The CD44 gene comprises a plurality of exons, 10 exons of which, located next to one another, are spliced from the pre-mRNA in different arrangement during mRNA generation. In rat carcinoma cells it was detected that metastasizing variants carry exons 4 to 7 or 6 to 7. With the aid of antibodies against the exon 6-encoded part of the protein it was possible to suppress metastasis efficiently (Sherman, L., et al. (1996) Curr. Top. Microbiol. Immunol. 213: 249-269).
Incorrect splicing may lead to strongly developed phenotypes of the affected organism. Thus it is known that a point mutation in a xcex2-globin intron may lead to a xcex2+ thalassemia. The point mutation produces an incorrect splicing location which leads to a modified reading frame and to preliminary termination of the peptide chain (Weatherall, D. J. and Clegg, J. B. (1982) Cell, 29, 7; Fukumaki, Y. et al. (1982) Cell, 28, 585). In Arabidopsis thaliana mutants, for example, a point mutation at the 5xe2x80x2 splicing site of the phytochrome B gene leads to incorrect expression of the gene. This modification makes it impossible to remove an intron whose sequence includes a stop codon. Development of the plants is disrupted, since the gene is involved in phytomorphogenesis (Bradley, J. M. et al. (1995) Plant Mol. Biol., 27, 1133).
Up until now, only a few studies have been known, which have described influencing of splicing processes in the cell. Thus it is possible, with the aid of antisera or monoclonal antibodies against components of the splicing apparatus, to prevent generation of mature mRNA (Padgett, R. A. et al. (1983) Cell, 35, 10; Gattoni, R. et al. (1996) Nucleic Acid Res., 24, 2535).
The NS1 protein which is encoded by the influenza virus genome may likewise interfere in splicing by binding to U6 snRNA. The protein binds to nucleotides 27-46 and 83-101 of human U6 snRNA and thus prevents U6 from being able to interact with the partners U2 and U4 during the splicing process (Fortes, P. et al. (1994) EMBO J., 13, 704; Qiu, Y. and Krug, R. M. (1995) J. Virol., 68, 2425). Moreover, the NS1 protein also seems to prevent export from the nucleus by binding to the polyA tail of the mRNA formed (Fortes, P. et al. (1994), supra; Qiu, Y. and Krug, R. M. (1994), supra). Similar actions are described for a gene product of the Herpes simplex virus type 1 genome. In in vitro experiments, the protein ICP27 was able to effectively prevent splicing of a model RNA (xcex2-globin pre-mRNA) (Hardy, W. R. and Sandri-Goldin, R. M. (1994) J. Virol., 68, 7790). In addition, peptides which have been generated from the C-terminal domain of the large subunit of RNA polymerase II seem likewise to be able to interfere in splicing processes (Yurvey, A. et al. (1996) Proc. Natl. Acad. Sci USA, 93, 6975; WO97/20031). The incorporation of artificial nucleotide analogs (5-fluoro-, 5-chloro- or 5-bromouridine) into the mRNA to be spliced may likewise lead to inhibition of the splicing process in vitro (Sierakowska, H. et al. (1989) J. Biol. Chem., 264, 19185; Wu, X. P. and Dolnick, B. (1993) Mol. Pharmacol., 44, 22).
A number of further studies relates to the action of antisense oligo-nucleotides on splicing. Thus, the ratio of two different splicing products of the c-erb oncogene mRNA (c-erbA-alpha 1 and 2) from rats seems to be regulated by another mRNA, rev-ErbA-alpha. Rev-ErbA-alpha is a naturally occurring antisense RNA which pairs with c-erbA-alpha 2 mRNA but not with c-erbA-alpha 1 mRNA. An excess of rev-ErbA-alpha mRNa constructs which were complementary to the 3xe2x80x2 splicing site made it possible to effectively inhibit splicing of c-erbA-alpha pre-mRNA to c-erbA-alpha 2 mRNA (Munroe, S. H. and Lazar, M. A. (1991) J. Biol. Chem. 266 (33), 22083). Furthermore it was shown that generation of antisense RNA which bind to intron sequences of the mRNA to be spliced may likewise inhibit splicing (Volloch, V. et al. (1991) Biochem. Biophys. Res. Comm., 179, 1600). Hodges and Crooke were able to show that for weakly recognized splicing sites oligonucleotide binding is sufficient in order successfully to stop splicing. If, however, preferably recognized splicing sites are incorporated into the constructs, oligonucleotides which in addition can cause activation of RNase H are required (Hodges, D. and Crooke S. T. (1995) Mol. Pharmacol., 48, 905). A more detailed analysis of the pre-mRNA sequences required for splicing showed that 19 nucleotides upstream from the branching point adenosine and 25 nucleotides around the 3xe2x80x2 and 5xe2x80x2 splicing site are suitable sequences for generating antisense RNAs (Dominski, Z. and Kole, R. (1994) Mol. Cell Biol., 14, 7445). Studies with antisense molecules were carried out in particular for inhibition of viruses. Viruses which effect higher organisms often carry intron-containing genes in their genome. Thus it was possible to show that antisense oligo-nucleotides against the 3xe2x80x2 splicing site of the immediate early pre-mRNA 4/5 gene of Herpes simplex virus was able to inhibit virus replication in Vero cells (Iwatani, W. et al. (1996) Drug Delivery Syst., 11, 427).
The splicing mechanism is studied in general firstly by preparing mRNA by in vitro transcription. To this end, genetic constructs from viruses, for example adenoviruses, or cellular structural genes are used. mRNAs of this kind include all important structural elements which are necessary for recognition of the mRNA by the spliceosome and for the splicing process. Generally, the mRNA is radiolabelled in order to make it possible, after fractionation on a denaturing urea polyacrylamide gel, to evaluate, owing to the characteristic band pattern, whether a splicing reaction has occurred or in which reaction step a disruption has occurred. However, test systems of this kind are very time-consuming and labor-intensive and are therefore not suited to the systematic finding of substances which can modulate splicing.
It was therefore an object of the present invention to find a test system which makes it possible to study in a simple and effective manner a large number of compounds from chemical or natural substance libraries for their action on splicing of nucleic acids (high throughput screening).
Surprisingly, it has now been found that a test system with a gel-free detection system for detecting a splicing reaction is suitable to overcome the above-described disadvantages of the conventional test system and is thus suitable for high throughput screening, for example in a robot system.
The present invention therefore relates to a test system comprising
(a) one or more identical or different immobilized nucleic acid(s) having at least one spliceable nucleic acid.
(b) at least one gel-free detection system for detecting a splicing reaction, where appropriate
(c) at least one composition comprising splicing components, and preferably
(d) suitable detection probes, and, where appropriate,
(e) further aids.
To provide a gel-free test system for studying splicing processes, the nucleic acid to be studied has to be immobilized to a solid phase. The nucleic acid may be immobilized, for example, covalently, by introducing particular structural elements, for example aptamers, into the nucleic acid to be spliced and using binding partners for said structural elements or by hybridization.
Additionally and advantageously, a suitable probe which facilitates detection of the splicing which has or has not occurred has to be generated for the gel-free test system. Said probe may be, for example, an oligonucleotide used for hybridization to the nucleic acid to be studied or a binding partner which binds to structural elements introduced into the nucleic acid to be studied.
Therefore, the gel-free detection system advantageously comprises at least one probe. The probe is in particular a nucleic acid complementary to the spliceable nucleic acid, a low molecular weight compound which binds the spliceable nucleic acid, and/or a peptide or protein which binds the spliceable nucleic acid.
In a preferred embodiment the spliceable nucleic acid comprises at least two exons which are separated by at least one intron.
For example, the complementary nucleic acid is complementary to at least one intron, to at least one exon and/or to at least one exon/intron transition site and/or to the exon/exon boundary generated after fusion of the two exons. The complementary nucleic acid here serves as a probe for detecting a splicing reaction.
Thus it is possible, for example, to detect the intron liberated during the splicing reaction by means of the gel-free detection system, and this leads to the conclusion that both steps of the splicing reaction have been completed. Alternatively, it is possible to determine on the basis of a suitable detection system, for example a nucleic acid complementary to an exon/intron transition site, whether the exon has been removed from the intron during the splicing process; this may provide information about whether the first splicing reaction has taken place at the 5xe2x80x2 end of the intron and/or the second splicing reaction at the 3xe2x80x2 end of the intron. Other forms of detection are illustrated in detail hereinbelow.
In a particularly preferred embodiment, the probe is a low molecular weight compound, for example theophylline, xanthine or an aminoglycoside such as tobramycin. If, for example, the spliceable nucleic acid or a nucleic acid of the gel-free detection system, which is complementary to the spliceable nucleic acid, comprises an xe2x80x9captamer structurexe2x80x9d, i.e. a binding sequence for this kind of binding partner (see, for example, Jenison, R. D. et al (1994) Science 263, 1425-1429, Hamasaki, K. et al. (1998) Biochem. 37, 656-663 or Kiga, D. et al. (1998) Nucleic Acids Res., 26 (7), 1755-1760), then the splicing process can be detected particularly easily via the binding partner.
In another particularly preferred embodiment, the binding partner may be a nucleic acid binding protein, in particular an iron responsive element binding protein (IBP), which recognizes a recognition sequence for a nucleic acid binding protein, in particular an iron responsive element (IRE). A splicing process which may have occurred is detected here via the nucleic acid binding protein.
The described interactions between binding partner and structural element in the nucleic acid (low molecular weight compound and aptamer, IBP and IRE, oligonucleotide and sequence in the nucleic acid) are likewise suitable for immobilizing the nucleic acid to be studied and capable of being spliced to a solid phase. To this end, the binding partner must be anchored to the solid phase in a suitable manner. In this connection it is possible, for example, to bind the binding partner covalently to the solid phase. Furthermore, coupling of biotin to the nucleic acid and the use of (strept)avidin bound to the solid phase are suitable for anchoring the nucleic acid. Said anchoring may also be achieved, for example, by using antibody/antigen interactions.
Generally, the probe comprises a label, for example a radiolabel, a label by fluorescent dyes, by biotin, by digoxigenin and/or by antibodies. The label is preferably attached to the ligand, for example to the complementary nucleic acid, to the low molecular weight compound or to the nucleic acid binding protein. It is possible, in particular with the aid of fluorescent dyes, to determine in a simple and, in automated systems, rapid manner, whether it is possible for a splicing reaction to progress undisturbed, for example in the presence of at least one substance to be studied, for example by removing the binding partner of the probe in the nucleic acid during the splicing process.
In another preferred embodiment, the spliceable nucleic acid and the probe-binding nucleic acid are linked to one another. This has the advantage of it being possible to detect the splicing reaction directly, for example via liberation of the probe-binding nucleic acid. In this embodiment, the probe-binding nucleic acid is preferably a nucleic acid which can bind a low molecular weight compound, for example an xe2x80x9captamerxe2x80x9d, and/or a nucleic acid binding protein. Since, generally, particular structural elements of the nucleic acids are responsible for binding of probes of this kind, the probe-binding nucleic acid is abbreviated in the preferred constructs below to xe2x80x9cSExe2x80x9d for structural element, where xe2x80x9c3xe2x80x2 regionxe2x80x9d is a nucleic acid section at the 3xe2x80x2 end of the nucleic acid:
1. Constructs with a structural element (SE) introduced into the 5xe2x80x2 exon:
---T7 promoter---SE---exon1---intron---exon2---3xe2x80x2region---
---T7 promoter---exon1---SE---exon1---intron---exon2---3xe2x80x2region---
2. Construct with a structural element introduced into the intron:
---T7 promoter---exon 1---intron---SE---intron---exon2---3xe2x80x2region---
3. Constructs with a structural element introduced into the 3xe2x80x2 exon:
---T7 promoter---exon1---intron---exon2---SE---exon2---3xe2x80x2region---
---T7 promoter---exon1---intron---exon2---SE---3xe2x80x2region---
---T7 promoter---exon1---intron---exon2---3xe2x80x2region---SE---3xe2x80x2region---
---T7 promoter---exon1---intron---exon2---3xe2x80x2region---SE---
4. Constructs with combinations of the constructs listed in 1-3.
5. Constructs with various structural elements:
---T7 promoter---exon1---SE1---intron---exon2---SE2---3xe2x80x2region
---T7 promoter---exon1---SE1---exon1---intron---SE2---intron---exon2---SE3---3xe2x80x2region---
The constructs in 1. here in particular serve to detect whether it was possible to remove exon1 from the intron sequence during the splicing process. The construct in 2. serves to detect directly a removed intron sequence. The constructs under 3. serve to detect whether it was possible to remove exon2 successfully from the intron sequence. A combination of the constructs according to 4. serves to detect the individual intermediates and end products during the splicing process. Exon1 is generally an exon located 5xe2x80x2 of the intron and exon2 is generally an exon located 3xe2x80x2 of the intron.
The constructs under 5. comprise various additional recognition sequences which may, on the one hand, relate to various detection systems and which may, on the other hand, facilitate binding of the nucleic acid to a solid phase via their binding partners. Thus it is possible, for example, to introduce a probe-binding nucleic acid into the 3xe2x80x2 region for immobilizing and, at the same time, another probe-binding nucleic acid into exon1 (see 5., first construct) for detecting the splicing process. Moreover, it is possible to introduce, for example, three different nucleic acids which serves, firstly, to immobilize the entire nucleic acid and, secondly, to detect the removed intron and to detect the linkage of exon1 to the remaining nucleic acid (see 5., second construct). The location of the individual probe-binding nucleic acids listed, by way of example, under 5. may vary according to the above-described constructs under 1.-4. In addition, the exact location of the individual probe-binding nucleic acids is variable.
In another embodiment, therefore, the nucleic acid, preferably the spliceable nucleic acid, in particular a nucleic acid according to any of the above-described embodiments, is bound to a solid phase directly covalently or indirectly via a structural element and binding partner of the structural element or by means of hybridization.
The direct covalent binding may take place, for example, via the 3xe2x80x2 terminal cis-diol group of the ribose backbone of the nucleic acid. It is possible, for example, to bind an RNA to hydrazine groups of the solid phase after periodate oxidation of the vicinal 2xe2x80x2, 3xe2x80x2 hydroxyl groups of the 3xe2x80x2 terminal ribose. For indirect binding suitable linkers such as, for example, biotin linkers or dicarboxylic acid linkers are also suitable. As already stated above, however, the nucleic acid may also be bound via a binding partner, for example via theophylline, xanthine or an aminoglycoside such as tobramycin and/or via a nucleic acid binding protein such as, for example, IBP.
Suitable for immobilizing a nucleic acid to a support-bound ligand is, for example, in the case of theophylline as support-bound binding partner, the theophylline aptamer Th (Kd=0.9 xcexcM) (see, for example, Jenison, R. G. et al. (1994) supra) or in the case of tobramycin as support-bound binding partner, the minimal version of the tobramycin aptamer To (Kd=0.2 xcexcM) (Hamasaki, K. et al. (1998) supra). The sequences of the two aptamers are preferably:
Th: AAGUGAUACCAGCAUCGUCUUGAUGCCCUUGGCAGCACUU (40mer, SEQ ID No.:1)
To: GGCUUAGUAUAGCGAGGUUUAGCUACACUCGUGCUGAGCC (40mer, SEQ ID No.:2)
The solid phase here is, for example, ceramic, metal, in particular noble metal, glass, plastic or polysaccharides, for example an agarose polymer.
However, probe-binding nucleic acids, for example aptamers, are also suitable for binding labeled probes, as a result of which it is possible, for example, to detect and also quantify the aptamer-containing nucleic acid. It is possible, for example, to react tobramycin with commercially available, NH2-reactive fluorescent dyes (Wang, Y. et al. (1996) Biochemistry 35, 12338-12346). In the case of theophylline, preference is given to preparing a 1-aminoalkyl or 1-thioalkyl derivative of 3-methylxanthine, which can bind to the aptamer (Jenison, R. D. et al. (1994), supra).
According to the present invention, the spliceable nucleic acid is any nucleic acid which can be spliced, preferably an RNA, for example in the form of a xe2x80x9cpre-mRNAxe2x80x9d or in the form of a DNA comprising RNA sections. If the RNA is to comprise additional probe-binding sequences as already described in more detail above, it is advantageous if said probe-binding sequences are located on both exon sides at least approx. 25 nucleotides from the particular splicing site, on the intron side at least approx. 17 nucleotides from the branchpoint and/or at least approx. 7 nucleotides from the 5xe2x80x2 splicing site. This generally ensures that the additional probe-binding sequences cannot disturb the splicing reactions.
An example of a spliceable nucleic acid, which is suitable for splicing in the human system, is the MINX model pre-mRNA (MINX=miniature wild type substrate; Zillmann, M., Zapp, M. L., Berget, S. M. (1998), Mol. Cell. Biol., 8:814-21). It is preferably possible to introduce into the MINX-encoding DNA a further probe-binding nucleic acid suitable for detection or immobilization, as already described above, with the restriction enzyme cleavage site preferably being retained. This makes it possible to incorporate, if required, in a further cloning step another identical or different probe-binding nucleic acid for detection or for immobilization, in order to be able to enhance the fluorescence signals or to tighten binding to the solid phase.
Pre-mRNA is immobilized, for example, by inserting the Th or To aptamers at the 3xe2x80x2 end of pre-mRNA exon2 and covalently binding the corresponding binding partner to the solid phase. The corresponding coding nucleic acid sequences are depicted in FIGS. 1A (SEQ ID NO:3) and 1B (SEQ ID NO:4).
In this context, for example, the appropriate aptamer sequence is inserted as DNA oligonucleotide into the BamHI cleavage site of the coding Minx DNA, i.e. between positions 219 and 220 of the corresponding Minx pre-mRNA.
As already mentioned above, it is also possible to insert probe-binding nucleic acids into the pre-mRNA intron structure, said nucleic acids being liberated by splicing and thus being absent in the, for example immobilized, mRNA. Constructs of this kind are therefore suitable for detecting an inhibition of splicing in the first step, i.e. opening of mRNA and lariat formation, or in the second step, i.e. removal of the lariat. In the case of an inhibition of the splicing process, the probe-binding nucleic acids would not be removed from the pre-mRNA and could therefore be detected, for example, after immobilization of the pre-mRNA. Examples of suitable nucleic acid constructs are depicted in the form of their coding sequences in FIG. 2A (SEQ ID NO:5) and FIG. 2B (SEQ ID NO:6).
To this end, for example, the appropriate aptamer sequence is inserted as DNA oligonucleotide into the PstI cleavage site of the coding Minx DNA, i.e. between positions 88 and 89 of the corresponding pre-mRNA.
As already described in more detail above, the spliceosome opens in the first splicing step the linkage between exon1 and intron at the 5xe2x80x2 splicing site of the intron. Only in the second splicing step are exon1 and exon2 covalently linked. As a result, exon1 is no longer linked to the mRNA during the first step of the splicing reaction and is thus removable from the splicing reaction. In connection with constructs which have, for example, an aptamer structure in the intron, it is therefore possible to make a statement on whether, for example, an inhibition has occurred in the first splicing step. If, for example, two different aptamers which recognize different probes are incorporated at the 5xe2x80x2 end of exon1 and into the intron of the pre-mRNA, then it is possible to follow both the first splicing step and the second splicing step in a test system. Examples of suitable nucleic acid constructs are depicted in the form of their coding sequences in FIG. 3A (SEQ ID NO:7) and FIG. 3B (SEQ ID NO:8).
To this end, for example, the appropriate aptamer sequence is inserted as DNA oligonucleotide into the EcoRI cleavage site of the coding Minx DNA, i.e. between positions 9 and 10 of the corresponding pre-mRNA.
For studies in the yeast system it is possible, for example, to start from the pre-mRNA for yeast U3 (Mougin, A. et al. (1996), RNA, 2: 1079-93) and to incorporate, for example, suitable aptamers such as, for example, the above-described theophylline or tobramycin aptamers. Examples of suitable nucleic acid constructs are depicted in the form of their coding sequences in FIGS. 4A to 4C (SEQ ID NOs:9, 10 and 11, respectively).
The nucleic acid construct according to FIG. 4A (SEQ ID NO:9) is prepared, for example, by inserting a suitable aptamer as DNA oligonucleotide into the SacII cleavage site of the coding U3 DNA, i.e. between positions 22 and 23 of the pre-U3 RNA.
The nucleic acid construct according to FIG. 4B (SEQ ID NO:10) is prepared, for example, by inserting a suitable aptamer as DNA oligonucleotide into the BstNI cleavage site of the coding U3 DNA, i.e. between positions 105 and 106 of the pre-U3 RNA.
The nucleic acid sequence according to FIG. 5A (SEQ ID NO:12) is an example of a nucleic acid sequence with an iron responsive element (IRE) which is suitable for studies in the human system. The IRE is inserted here at the 3xe2x80x2 end of exon2 in analogy to the above-described aptamers.
For studies in the yeast system it is possible to insert an IRE element, for example, at the 3xe2x80x2 end of exon2 of pre-U3RNA, as depicted in FIG. 5B (SEQ ID NO:13).
The present invention therefore further relates to a spliceable nucleic acid splicing, as illustrated by way of example above, and to the use thereof for preparing a test system.
The studies of the individual splicing reactions with the aid of a test system of the invention are commonly carried out by using a composition comprising the individual splicing components, preferably small nuclear ribonucleoprotein particle (snRNP) components and non-snRNP components. The snRNP components particularly comprise U1, U2, U4, U5 and/or U6 proteins. Preference is given in particular to using appropriate cell extracts, in particular eukaryotic cell extracts, for the studies. It is possible, for example, to obtain the cell extracts from animal cells, in particular mammalian cells, especially Hela cells, in particular from nuclear extracts of HeLa cells or cell extracts of fungi, in particular yeasts, according to methods generally known to the skilled worker (see examples). The cell extracts generally comprise all important factors in order to be able to carry out splicing in vitro.
It is essential for carrying out the studies to use further aids such as, for example, buffer solutions, stabilizers and/or energy equivalents, in particular ATP.
The present invention therefore also relates to a method for preparing a test system in which at least one immobilized spliceable nucleic acid and at least one gel-free detection system and also, where appropriate, at least one composition comprising splicing components and, where appropriate, further aids are combined. Preferred embodiments of the individual components have already been described in more detail above.
The present invention further relates to a method for finding an active substance, which comprises
(a) incubating one or more identical or different immobilized nucleic acid(s) with at least one spliceable nucleic acid sequence in the presence of at least one substance to be studied and at least one composition comprising splicing components and, where appropriate, further aids under suitable conditions, and
(b) detecting the splicing product which may have formed by means of a gel-free detection system.
Preferred individual components of the method of the invention have already been described in more detail above.
The active substance here may be a pharmaceutically active compound, a fungicide, a herbicide, a pesticide and/or an insecticide, and is preferably an antibiotic. The substance to be studied is generally a naturally occurring, a naturally occurring and chemically modified, and/or a synthetic substance. The method of the invention makes it possible in particular to screen xe2x80x9ccombinatorial substance librariesxe2x80x9d in a simple and rapid manner.
In the introduction of the description it was already indicated that various disorders can be attributed to a disruption of the splicing mechanism. The present invention is therefore also suitable for diagnosing a disorder.
The present invention therefore further relates to a method for diagnosing a disorder, which comprises
(a) incubating one or more identical or different immobilized nucleic acid(s) with at least one spliceable nucleic acid in the presence of at least one composition comprising splicing components and, where appropriate, further aids under suitable conditions, and
(b) detecting the splicing product which may have formed by means of a gel-free detection system.
The disorders to be diagnosed here are preferably genetic disorders, cancers and/or viral diseases, in particular Graves"" disease, spinal muscular atrophy, xcex2xe2x80x2 thalassemia, cancers related to the c-erb oncogene, hepatitis C infections and/or Herpes simplex virus infections. The composition comprising splicing components may in this case be a treated or untreated tissue sample of a patient, for example.